1. Field of the Invention
The present invention relates generally to compounds terminated with substituted silyl groups, to oligomers formed from such compounds, to polymer-ceramic networks formed from such oligomers and compounds, and to processes for forming such compounds, oligomers, and polymers.
2. Description of the Background Art
High-temperature adhesives, coating, and encapsulants are used for a variety of aerospace applications, such as aircraft, missile, and spacecraft structures, where materials must survive temperatures as high as 1000.degree. C. Presently used materials with high temperature capability are generally brittle and very difficult to process, and many degrade in the presence of atmospheric moisture.
One approach to develop high-temperature materials has been to physically blend organic and inorganic materials to produce composite structures which have both the flexibility of organic polymers and the compressive strength of ceramics. The art is replete with the addition of various polymeric resins (most significantly, polymethacrylate type resins) into concrete. The resulting concrete has greatly improved fracture toughness. Alternatively, various inorganic polymers and mixtures of such polymers have been heated at high temperatures to cause the polymers to coalesce into the ceramic fibers. In addition, organic monomers such as the methacrylates have been blended into fluid cement mixtures and then the monomers have been polymerized in situ in concurrence with the concrete formation process. None of these variations was designed to yield materials which can be used in the 400.degree. to 1000.degree. C. temperature range, as required in many structural and adhesive applications.
The state-of-the-art approach to improve the high temperature performance of organic polymer resins has included the end-capping of a variety of oligomers such as phenylene, imide, ether-ketone-sulfone, phenylquinoxalines, and phenyl-as-triazines with reactive ethynyl end groups. The purpose of using such reactive end groups was to provide a mechanism by which the oligomers could undergo thermally induced chain extension and crossliking reactions. However, the final cured resins have been found to have less thermo-oxidative stability than the corresponding polymers which did not contain the ethynyl groups, as disclosed by P. M. Hergenrother, Macromolecules, Vol. 14, 1981, pages 891 et seq. That the ethynylated oligomers cannot produce cured resins for application above 371.degree. C. was apparently demonstrated by F. E. Arnold and F. L. Hedberg in Preprints, American Chemical Society Division of Polymer Chemistry, Volume 21, 1980, pages 176 et seq. The thermo-oxidative instability is ascribed to the presence of non-aromatic end products as a result of the ethynyl groups undergoing thermally induced cross-linking. In fact, solid state carbon-13 NMR studies showed that less than 30 percent of the ethynyl groups had undergone thermal cyclic trimerization to yield stable aromatic rings (M. D. Sefcik, E. 0. Stejskal, R. A. McKay, and J. Schaefer, Macromolecules, Vol. 12, 1979, pages 423 et seq. U.S. Pat. No. 4,528,216 discloses a process for forming heat resistant resin films by mixing a polyimide resin precursor solution, i.e. a polyamic acid solution, with an organosilicic compound solution which comprises a silicon compound of the formula R.sub.n Si(OH).sub.4-n, an additive such as a glass forming agent, and an organic binder in an organic solvent, by depositing the mixture on a silicon substrate and heating up a temperature gradient of 80.degree. -500.degree. C. for one hour. Optionally, a minor portion of the silicon compound may be replaced with PO(OH)(OR).sub.2. The product is characterized as having Si--O--Si and polyimide molecular structures and is referred to as a "silicon polyimide resin film." The product formed by the process of U.S. Pat. No. 4,528,216 comprises a mixture of a polyimide component and a Si--O--Si component.
With regard to the ceramic component of prior art polymer-ceramic composites, the feasibility of preparing refractory materials at very low temperatures has produced materials stable well beyond 700.degree. F. (371.degree. C). For example, aluminum phosphate-based glass material is refractory up to 1600.degree. C., at which point aluminium phosphate begins to decompose. The processing is typically carried out at low temperature and heat treatment requires only temperatures as low as 100.degree. C., as disclosed by Birchall and Kelly, Sci Amer., Vol. 248, No. 5, 1983, pages 104 et seq. The related silicon alkoxide-based materials, made also at low temperatures, have been the subject of intense studies, as disclosed by B. E. Yoldas, J. Mat. Sci., Vol. 12, 1977, pages 1203 et seq. and J. Mat. Sci., Vol. 14, 1979, pages 1843 et seq. In principle, these types of glass materials can be reinforced with polymer fibers. The low temperature aspect in processing allows the preparation of polymer-ceramic composite materials based on many otherwise hard-to-process high temperature polymers.
Modern ceramic processing places strong emphasis on the chemical processes which lead to particles with controllable purity, uniformity in size and submicronic dimensions. These two features will ultimately lead to attainment of maximum theoretical density. There are two fundamental methodologies to address this micromorphology requirement. The solgel process, which has been widely used in making fine powder of various metal oxides, involves the conversion of a sol to gel by means of polymerization. On gradual heating, the gel is converted to fine powder or glass. This method can produce extremely homogeneous mixtures of two or more components, since mixing of these ingredients takes place at the molecular level. An alternative method (referred to as Barringer's procedure) is to generate a colloidal dispersion of the metal oxide, followed by precipitation of essentially uniform submicron particles. As previously mentioned, the superfine nature and uniform size of the powder are conducive to producing ceramic structures with greater than 99% of the theoretical maximum density.
With regard to high-temperature polymers, compounds possessing the hexafluoroisopropylidene group are well known for their thermo-oxidative stability. Their diphenyl derivatives in particular are known for their contribution to the optical transparency of polymers containing them. They are also thermo-oxidatively stable. However, these materials do not have the dimensional integrity and solvent resistance that are desired for many uses. U.S. Pat. No. 4,503,254 assigned to the present assignee, discloses the conversion of 2,2-bis(4-hydroxyphenyl)hexafluoropropane (commercially available as Bisphenol AF) to 2,2-bis(4-halophenyl)hexafluoropropane which can be used to produce a precursor material for subsequent derivatization to produce thermally stable resins for use in high temperature structural composites. Similarly, the art is replete with other substituents besides halogen being substituted on the phenyl group of Bisphenol AF to produce other pertinent intermediates. Some of these other substituents include ethynyl, phenylethynyl, epoxy and amides. These compounds have typically been used as intermediates in forming other resins.
Thus, in view of the previous discussion, it can be seen that a need exists for a polymer which is stable at high temperature, moisture insensitive, tough and easily processible.